In the field of display technology, the Active Matrix Organic Light Emitting Diode (AMOLED) display apparatus has been drawing people's attention gradually due to its many advantages such as being super thin, good quake resistance, large visual angle, short response time, good performance at low temperature, high emission efficiency, capability of being made into flexible display, etc. FIG. 1(a) is a basic pixel circuit structure for driving an AMOLED by N type Thin Film Transistors (TFTs) as known in the art, and FIG. 1(b) is a basic pixel circuit structure for driving an AMOLED by P type TFTs as known in the art. In FIG. 1(a) and FIG. 1(b), VDATA is a data level signal, VSCAN is a scan signal, VDD is a high voltage level signal, VSS is a low voltage level signal, T1 and T2 are TFTs, C1 is a capacitor, and D1 is a LED. The circuits in FIG. 1(a) and FIG. 1(b) is applicable to all kinds of transistors, including the depletion type TFT. However, these pixel circuits do not have a threshold voltage compensation function and thus cannot solve the threshold voltage uniformity problem and the OLED's driving light emission uniformity problem caused by the process uniformity problem.
The oxide TFT is the development direction of the large AMOLED. The oxide TFT devices mostly have characteristics of the depletion type, that is, the threshold voltage of the N type is negative. FIG. 2 is the Ids-Vgs characteristic curve of the N type depletion TFT as known in the art, where Ids is the current between the TFT drain and source, and Vgs is the voltage between the TFT gate and source. It can be seen from FIG. 2 that the most distinctive feature of the N type depletion TFT is that the threshold voltage is less than 0.
FIG. 3(a) is an AMOLED pixel driving circuit with a threshold voltage compensation function commonly seen in the art, wherein the gate of the driving TFT T1 is g, the source is s, the drain is d, C is a storage capacitor, D1 is a LED, VINI is an initial level signal, VDD is a high voltage level signal, VSS is a low voltage level signal, DATA is a data level signal, Gn is a gate control signal of the TFTs T2 and T4 respectively, Gn-1 is a gate control signal of the TFT T5, and EM is a gate control signal of the TFTs T3 and T6 respectively. In a voltage programming stage, as shown in FIG. 3(b), the connection of T1 to the high voltage level signal VDD and the low voltage level signal VSS is cut off first, the terminal of the storage capacitor connected to the gate g of T1 is charged to the initial level voltage VIN1, the terminal of the storage capacitor connected to the source s of T1 is charged to the data level voltage VDATA, and then the gate g of the driving TFT T1 is connected to the drain d (i.e. turning on the transistor T4 in FIG. 3(a)) to form a diode connection manner to perform discharge, that is, to discharge the voltage between the two terminals of the storage capacitor from VIN1-VDATA to the subthreshold turning on state VTH, where VTH represents the threshold voltage of T1. When the driving TFT has a general characteristic of the enhancement type, the threshold voltage is positive, as shown in FIG. 4(a), and the voltage between the two terminals of the storage capacitor can be discharged to VTH normally to realize the threshold voltage compensation. However, when the driving TFT has the depletion type characteristic, the threshold voltage is negative, as shown in FIG. 4(b), when the voltage between the two terminals of the storage capacitor is discharged through the driving TFT connected to the diode, and the source-drain voltage of the driving TFT becomes zero cut-off, the voltage between the two terminals of the storage capacitor has not yet to be discharged to reach the subthreshold turning on state, in other words, the voltage between the two terminals of the storage capacitor is 0 but not VTH (VTH<0). Therefore, the pixel driving threshold voltage compensation fails. Vds in FIG. 4(a) and FIG. 4(b) represents the voltage between the TFT drain and source.
To sum up, if a conventional AMOLED pixel driving circuit design of N type TFTs is applied to the depletion type TFT, when the threshold voltage is compensated for by using the diode connection manner, the threshold voltage compensation function would fail, because the threshold voltage is negative, and thus before the TFT enters the subthreshold saturation cutoff, the source-drain voltage becomes zero and the TFT is cut off in advance.